In a normal display of TFT-LCD (Thin Film Transistor Liquid Crystal Display) panel, a COF (Chip On Film) substrate is required to connect a PCB (Printed Circuit Board) and a liquid crystal panel, so that the signal can be transmitted to the panel from the PCB.
As shown in FIG. 1, welded lead lines 2 are disposed on a COF substrate 1, and the welded lead lines 2 are coated with a protective layer 3. Since copper has good thermal conductivity properties and excellent flexibility, copper is often used as the material of the welded lead line. The welded lead lines which remain uncoated with the protective layer on the COF substrate are used to connect terminal wires of the crystal panel.
In the module stage, after the completion of the welding of the COF substrate, a layer of tuffy resin is coated on a lateral side of the thin-film transistor of the liquid crystal panel for preventing the terminal wires of the liquid crystal panel from being corroded with H2O and chloride ions. However, due to the limitations of the manufacturing process, the tuffy resin is generally not coated on the back surface of the COF substrate. As shown in FIG. 2, therefore, portions of the welded lead lines on the COF substrate are exposed to air. Although copper has strong corrosion resistance, and is not easily corroded by water vapor, when ions are present in the environment, an electrochemical reaction occurs, and copper will still be corroded. This is particularly true when chloride ion are present, as chloride ions have strong properties of penetration and are easily hydrolyzed to acid, so as to greatly speed up the following electrochemical reaction, and cause point corrosion, pit corrosion, and even breakage of the exposed copper in severe cases, thereby affecting the signal transmission. The electrochemical reaction is as follows:
